1. Field of the Invention
The present invention generally relates to an optical scanning device and an image forming apparatus, and more particularly, to an optical scanning device including a plurality of scanning optical systems (optical scanners) each deflecting a light beam by oscillating a micro mirror, and an image forming apparatus, which are used in a laser beam printer, a digital plain-paper copying machine, a plain-paper facsimile and so forth.
2. Background of the Invention
Some of color laser beam printers, color copying machines, color facsimiles and so forth are each provided with a plurality of scanning optical systems for the purpose of increasing a rate of image writing or image forming. The invention described in Japanese Laid-Open Patent Application No 10-244708 is one of examples thereof, in which a plurality of scanning optical systems are provided, each of which performs an optical scanning by using a polygon scanner. Additionally, there is also provided a synchronous revolution controlling means for adjusting a revolving speed of each of polygon motors.
Besides, as a new type of optical scanning device, an optical scanning device comprising a deflecting unit performing an optical scanning by oscillation is being studied. This deflecting unit uses a sine-wave oscillation. Additionally, another optical scanning device is being studied in which a mechanical resonance frequency of the deflecting unit is variable.
An optical scanning device deflecting a pencil by a deflector utilizing reflection, and using the deflected pencil for scanning, is well known conventionally. A polygon scanner using a polygon mirror revolving at a constant speed is widely used as the deflector. However, since the polygon scanner makes a scale of the device large, there are problems, such as a banding due to vibration, a temperature rise, noises, and increased power consumption.
Also, an optical scanning device of another type is known, in which a plurality of scanning optical systems are arranged in parallel in a main scanning direction. This type provides advantages as follows:
A compact optical scanning device capable of scanning a wide scan area.
Small-size scanning optical systems and scanning optical elements which facilitates a correction of wave aberration: this reduces a variation of a beam spot diameter due to component variations or errors in mounting components; thus, it becomes easy to reduce a beam spot diameter.
Also, it is known that a plurality of scanning optical systems are arranged in a sub-scanning direction so as to be applied in a multicolor image forming apparatus.
On the other hand, there is proposed a micro mirror that is provided with a resonance structure by using a micromachining technology, and is caused to oscillate in a sine-wave form so as to deflect a light. Using this micro mirror reduces a scale of a device as a whole, and largely alleviates the above-mentioned problems, such as a banding due to vibration, a temperature rise, noises, and increased power consumption.
However, a scanning frequency of the micro mirror oscillating in the sine-wave form depends on an intrinsic resonance frequency which may become a different value due to manufacture variation.
When scanning optical systems have different resonance frequencies, positions of scanning lines become different for each of the scanning optical systems. Thus, when a plurality of scanning optical systems are arranged in a main scanning direction, a dot position displacement occurs in a sub-scanning direction at a joint of scanning lines joined in the main scanning direction. Besides, when a plurality of scanning optical systems are arranged in the sub-scanning direction, a color displacement occurs in the sub-scanning direction.
Besides, a recent optical scanning device scanning with a light beam, such as a laser light, is used in an optical apparatus, such as a barcode reader, a laser printer, or a head mount display. As an optical scanning device of this type, there is proposed an optical scanner that oscillates a micro mirror utilizing a micromachining technology, as mentioned above.
FIG. 1A is a perspective view of an optical scanner used in a conventional optical scanning device. FIG. 1B is a sectional view of the optical scanner shown in FIG. 1A. This optical scanner comprises a mirror 1 supported by a rotary axis composed of two elastic members 3 as beams arranged in a line, a moving electrode 26 provided on the mirror 1, and a stationary electrode 25 provided on a stationary member 5 so as to oppose the moving electrode 26. The mirror 1 is oscillated back and forth by an electrostatic suction force generated between the moving electrode 26 and the stationary electrode 25, with the two elastic members 3 functioning as a twisting axis.
In the above-described optical scanner, a resonance frequency f0 of the mirror 1 is generally obtained by the following expression.f0=½π√{square root over ( )}(Kθ/I)  (Expression 1)
In this expression, I represents a moment of inertia of the mirror, and Kθ represents a spring constant determined by the two elastic members 3.
At this point, precisions of the elastic members 3 vary through a manufacturing process thereof. Accordingly, Kθ varies. Therefore, the resonance frequency f0 represented by the expression 1 also varies.
In an optical scanning device including a plurality of the optical scanners, when the resonance frequency f0 of the mirror 1 varies among the optical scanners, an oscillation angle θ of the mirror 1 shown in FIG. 1B varies, as a result of which a joint of images formed by one optical scanner and an adjacent optical scanner becomes visible, causing a problem of a deterioration of image quality.
Besides, for the purpose of solving the above-mentioned problems, such as a banding due to vibration, a temperature rise, noises, and increased power consumption, there is proposed a micro mirror that is provided with a resonance structure by using a micromachining technology, and is caused to oscillate in a sine-wave form, as mentioned above. Using this micro mirror reduces a scale of a device as a whole, and largely alleviates the above-mentioned problems.
However, the above-mentioned micro mirror moves alternately back and forth; thus, when a writing light beam is reciprocated in scanning, there is an inconvenience that scanning lines formed on a scanned surface come to have different intervals in height because the scanned surface moves in a sub-scanning direction while the beam is moved in a main scanning direction. This may cause image inferiorities, such as a density irregularity. Further, it is conventionally difficult to adjust a resonance characteristic of the micro mirror device.
To solve these problems, Japanese Laid-Open Patent Application No. 8-75475 discloses an optical scanning device using a deflector utilizing a resonance. This invention, which is intended to facilitate an adjustment of a resonance characteristic of a resonator used in the deflector, is a “resonator comprising an oscillation input part, a moving part, and an elastic deforming part having at least one resonant oscillation mode and coupling said oscillation input part and said moving part, characterized by including resonance characteristic adjusting means for adjusting a resonance characteristic” (claim 1 of the invention disclosed in Japanese Laid-Open Patent Application No. 8-75475).
However, according to the invention disclosed in Japanese Laid-Open Patent Application No. 8-75475, the resonance characteristic adjusting means is realized by separating a mass adjustment part by melting, which is unable to adjust the resonance characteristic effectively enough for an optical scanning device that requires a delicate and subtle adjustment.
Additionally, it is known that a plurality of scanning optical systems are serially arranged in a main scanning direction so as to compose a compact optical scanning device that can scan a wide scan area, and scanning optical systems and scanning optical elements are made small so as to facilitate a correction of wave aberration, which also reduces a variation of a beam spot diameter due to component variations or errors in mounting components; this arrangement certainly makes it easy to reduce a beam spot diameter.
Therefor, Japanese Laid-Open Patent Application No. 11-95152 discloses an invention in which two scanning optical systems are serially arranged in a main scanning direction, and beams are caused to scan in respective directions parting from a joint of the two scanning optical systems.
However, when further reducing the size of an optical scanning device, simply arranging two scanning optical systems serially faces a limitation. Additionally, when an optical scanning device comprises a plurality of scanning optical systems, image deterioration is likely to occur because dots are displaced relatively in the vicinity of a joint of formed image areas. Further, when a writing light beam is reciprocated in scanning, there has been an inconvenience that the dot position displacement becomes large at the joint. Especially, when three scanning optical systems or more are serially arranged in a main scanning direction, this arrangement requires a further contrivance to avoid such dot position displacement.